title: Likelihood ratios to apply for nasal bone, ductus venosus and tricuspid flow at the 11-13 weeks' scan in down syndrome screening.
abstract: OBJECTIVE
  To assess the feasibility of nasal bone (NB), ductus venosus (DV) and tricuspid flow (TF) at the 11-13 weeks' scan, calculate likelihood ratios for each of the markers and evaluate their efficacy in expanded and contingent screening strategies for Down syndrome.
  
  
  METHODS
  NB, DV and TF were assessed in 11,261 singleton fetuses undergoing first trimester combined screening. For each marker, Down syndrome detection rate (DR), false positive rate (FPR), positive, negative and isolated likelihood ratios (PLR, NLR and iLR) were calculated. Likelihood ratios were multiplied to the combined test risk either to the entire population or to the intermediate risk group (expanded and sequential strategies, respectively).
  
  
  RESULTS
  Down syndrome was diagnosed in 101 pregnancies. Feasibility for marker assessment ranged from 71 to 97%, DRs for isolated markers from 20 to 54% and FPRs from 1.3 to 5.3%. PLR ranged from 10 to 15, NLR from 0.5 to 0.8 and iLR from 3.9 to 5.6. When ultrasound markers were added to both strategies, a significant FPR reduction was observed.
  
  
  CONCLUSIONS
  The application of NB, DV and TF likelihood ratios to the combined test risk, either in an expanded or contingent strategy, result in a FPR reduction.
authors.affiliation: Department of Maternal-Fetal Medicine (Institut Clínic de Ginecologia, Obstetrícia i Neonatologia), Hospital Clinic-IDIBAPS, University of Barcelona, Barcelona, Catalonia, Spain.
authors.forename: Miriam
authors.initials: M
authors.lastname: Illa
authors.forename: Raquel
authors.initials: R
authors.lastname: Mula
authors.forename: Marta
authors.initials: M
authors.lastname: Arigita
authors.forename: Maribel
authors.initials: M
authors.lastname: Grande
authors.forename: Anna
authors.initials: A
authors.lastname: Gonce
authors.forename: Virginia
authors.initials: V
authors.lastname: Borobio
authors.forename: Antoni
authors.initials: A
authors.lastname: Borrell
mesh_terms: D000328:Adult; D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005189:False Positive Reactions; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D016013:Likelihood Functions; D009295:Nasal Bone / Q000000981:diagnostic imaging*; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D012680:Sensitivity and Specificity